(a) Field of the Invention
This invention relates to fine particle size aluminum powder. More particularly, this invention relates to a fine particle size aluminum powder composition suitable for use as a sensitizer in water-based slurry-type blasting agents.
Fine particle size aluminum powders, containing particles generally passing a 325 mesh Tyler sieve, and including particles having sizes down to a few microns, have many uses: in paints, inks, and similar coating compositions are well established ones, whilst a newer one is as a sensitizer in slurry-type blasting agents. But all uses for particulate aluminum wherein fine size particles are present involve a major disadvantage: micron size aluminum powder presents a serious hazard being prone to dust explosions. This very real hazard has limited, and still does limit, the commercial use of such aluminum powders, especially if the operation in question can involve the presence, at some stage, of a dry aluminum powder.
For a number of uses, these problems can either be sufficiently mitigated, or indeed overcome, by ensuring that the powder is fully coated at all times with an organic liquid (this is commonly called a "wetted down" condition). It is to be noted that, generally speaking, water is not a suitable wetting down liquid, since fine powder aluminum reacts spontaneously with water to release hydrogen gas--this property is utilized in some uses for such powders, for example in preparing blown cements and concretes. Generally the wetting down liquid used is a hydrocarbon material, such as kerosene, or mineral spirits (a hydrocarbon material having a boiling point of about 150.degree. C.-190.degree. C.).
Whilst these wetted down systems are far less hazardous than a dry aluminum powder, they are not a complete solution to all of the handling problems, for two separate reasons. The first is that to an extent one problem is replaced by another: a dust explosion hazard is replaced by a flammability hazard. The second is that very often the presence of the hydrocarbon liquid used to wet down the powder cannot be tolerated, since it leads to complications in the end use contemplated. This difficulty becomes acute in water-based systems which contain little, or no, organic solvents. Representative aqueous systems in which the presence of even minimal amounts of hydrocarbons is undesirable are cement and concrete foaming systems, and water-based slurry type blasting agents.
Aluminum powder used to sensitize water-based slurry blasting agents must meet two requirements. First, the powder must be active enough to function as a blasting agent sensitizer, preferably without the assistance of a booster such as TNT or PETN: that is, it is desirable that the sensitized blasting agent slurry be cap-sensitive. Second, the aluminum powder used as the sensitizer must not be so reactive that it reacts with the water present in the slurry. In an extreme case, such reaction can result in total loss, by dissolution, of the sensitizing powder. In an intermediate case, such reaction both generates gas bubbles in the slurry, and also encrusts the particles of aluminum with an oxide layer. Both of these are conditions that can have dangerous consequences. If the sensitizer is lost, the placed slurry (e.g. in a bore hole in an open pit mine or quarry) will not detonate at all. If large gas bubbles and/or a heavily oxidised sensitizer are present then such a placed slurry may go "dead" and either will not explode at all, or will not detonate properly.
Thus for blasting agent sensitizer use, a fine particle size aluminum powder is required which is non-dusting, is non-reactive to water, is relatively free of hydrocarbon and which is reactive enough to promote detonation, preferably through being cap-sensitive. Against these desiderata has to be set the fact that the industrially most convenient process for preparing fine particle size aluminum, of a size suitable for use as a sensitizer, is to comminute aluminum in a hydrocarbon medium such as kerosene or white spirits.
(b) Description of the Prior Art
It has been proposed to overcome these problems, and thereby provide an aluminum powder composition capable of functioning adequately as a sensitizer in a water-based slurry type explosive, by ball milling the aluminum in a hydrocarbon medium and in the presence of a polyvinylpyrrolidone resin. The hydrocarbon used as the grinding medium can then be displaced by contacting the wetted down composition with a liquid which is either a solvent for, or will swell, the polyvinylpyrrolidone resins. Suitable liquids for this purpose are water, ethanol, ethylene glycol, diethylene glycol and formamide. By this procedure an aluminum composition fully capable of acting as a sensitizer in a water-based slurry type blasting agent is obtained. These compositions are effective in bore holes down to 3" in diameter. Such compositions contain from 5% to 20% by weight of polyvinylpyrrolidone, based on the weight of aluminum powder present in the composition. A preferred range of polyvinylpyrrolidone resin is from 10% to 15%.
However these compositions generally containing 10% to 15% of polyvinylpyrrolidone, have been found to have a disadvantage. It has been found in practice that, even when an organic solvent is used, rather than water, in the displacement step, adequate separation of the aluminum powder/polyvinylpyrrolidone composition from the hydrocarbon liquid used in the grinding step is difficult. In practice it has been found that at least 10% of the hydrocarbon is always left in the composition, which complicates its proper incorporation into a slurry.